Fiber lasers using quartz fibers, to which rare earth ions are added as the laser medium, and ceramic lasers using polycrystalline ceramic materials, to which rare earth ions are similarly added as the laser medium, have been developed as light sources for laser processes or medical use. In particular, a high concentration of the rare earth ions may be added to the polycrystalline ceramic materials. Accordingly, if a laser medium is excited by means of light, it may require a shorter length to absorb the excitation light. The aforementioned advantageous characteristics make the polycrystalline ceramic materials more attractive as materials used for a compact laser device, which provides very efficient quality beam.
Laser light sources using the aforementioned ceramic material are examined by numerous organizations.
FIG. 28 is a schematic view of an infrared laser light source disclosed in Patent Document 1 or a short pulse laser light source disclosed in Non-Patent Document 1 or 2. FIG. 29 is a schematic view of a disc laser light source disclosed in Patent Document 2. The conventional laser light sources shown in FIGS. 28 and 29 are described hereinafter.
In addition to researches about laser mediums using polycrystalline ceramic materials, researches for polarization control of a semiconductor laser have also been conducted. For example, a polarization control method of a semiconductor laser using a thin film structure on a semiconductor substrate has been publicly disclosed.
Since the ceramic laser medium is an isotropic medium, the polarization of the generated light becomes randomly directed. If the output light has to be a single polarization, half of the output becomes loss. Since the ceramic laser medium does not have an epitaxial growth structure of a thin film semiconductor material like a semiconductor laser element, polarization control is not effective if the polarization control depends only on formation of the thin film structure.
Insertion of a single polarizing element into a laser resonator is exemplified as a conventional polarization control to the ceramic laser medium. The insertion of the single polarizing element, however, results in enlargement of the laser system. In addition, the insertion of the single polarizing element means addition of a new component, which increases manufacturing cost of the laser system.    Patent Document 1: Japanese Patent Application Laid-open No. 2002-57388    Patent Document 2: Japanese Patent Application Laid-open No. 2007-299962    Patent Document 3: Japanese Patent Application Laid-open No. H11-54838    Patent Document 4: Japanese Patent Application Laid-open No. H11-330630    Non-Patent Document 1: Applied Physics Letters Vol. 77, No. 23, Page 3707    Non-Patent Document 2: Japanese Journal of Applied Physics Vol. 40, Page L552